The working group headed by Professor Klaus Dreisewerd and Dr Jens Soltwisch from the Institute of Hygiene at the University of Münster have developed a method which has improved the spatial resolution of MALDI mass spectrometry by around one nanometre. The researchers have named the new technique t-MALDI-2 (with “t” standing for transmission mode). It uses two specially adapted lasers: one generates a particularly small focus on the material removed, while the other produces the necessary signal enhancement for many biomolecules by up to several magnitudes—for example, for fat-soluble vitamins such as vitamin D, cholesterol or administered medication. Information on their precise distribution in cells and tissues can, among other things, help to produce a better understanding of disease and inflammation processes and show new strategies for treating them.
Left: how the t-MALDI-2-MS imaging method works. Right: an example, in which the complex structure of a mouse’s cerebellum is shown by means of the superimposition of three ion signals. © Nature Research/Marcel Niehaus
“The decisive improvement which our method offers, in comparison with established MALDI imaging methods, is based on the combination and extension of two technical methods previously in use”, explains Dr Marcel Niehaus, one of the two lead authors of the study. “For one thing, in the transmission geometry we irradiate our samples on the reverse side. This enables us to place high-quality microscope lenses very close to the sample, thus reducing the size of the laser dot. This is different from what is possible, for geometrical reasons, in standard methods—where the samples are irradiated from the direction of the mass analyser.” However, in the minute areas of the sample which are removed by the laser, there is only an extremely small amount of material available for the subsequent MS measurement. The second decisive step was, therefore, the use of a method (called MALDI-2) which the researchers had already introduced in 2015. The effect is that the so-called post-ionisation laser produces an increased transfer of the initially uncharged molecules to an ionic form.
In their study, published in Nature Methods, the researchers demonstrate the possibilities offered by their technology, taking the fine structures in the cerebellum of a mouse and using kidney cell cultures. “Our method could improve the future understanding of many processes in the body at molecular level”, says Professor Dreisewerd. “Also, established methods from optical microscopy—for example, fluorescence microscopy—could be merged with mass spectrometry imaging in a ‘multi-modal’ instrument”, he adds.
